The present invention relates to code-division multiple access (CDMA) radiotelephone communications.
CDMA is a method of spread spectrum digital communication in which a plurality of transmission channels are created by using spreading sequences for each channel that modulate the information bits to be transmitted. The spreading sequences operate at a chip rate higher than the data bit rate in order to a achieve spectrum spreading of the radio signal. Their self- and cross-correlation properties are adapted to enable the various channels to be multiplexed: they are generally pseudorandom sequences that are mutually orthogonal or quasi-orthogonal, taking chip values of -1 or +1.
The use of CDMA in the field of cellular radiotelephone is described in chapter I of the work "Mobile radio communications" by Raymond Steele, Pentech Press, London 1992, and also in the article "On the system design aspects of code division multiple access (CDMA) applied to digital cellular and personal communications networks" by A. Salmasi and K. S. Gilhousen, Proc. of the 41st IEEE Vehicular Technology Conference, St. Louis, Mo., 19-22 May 1991, and in U.S. Pat. No. 5,103,459.
For the downlinks, the multiplexed transmission channels are formed at the base station of each cell in the network. Each mobile station situated within the cell uses a special spreading sequence to recover, from the overall radio signal transmitted by the base station, the data bits that are addressed thereto. One of the downlink CDMA channels is a pilot channel over which the base station transmits a reference sequence which does not modulate information bits. Reception over the pilot channel enables the mobile stations to be synchronized and to estimate the response of the propagation channel in order to perform coherent demodulation on the other channels.
For the uplinks, each mobile station transmits a radio signal formed by using a particular spreading sequence. A correlation with this sequence enables the base station to recover the information bits transmitted by the mobile station, among the radio signals received from different mobile stations in the cell. There is no pilot channel in the signal transmitted by a mobile station. The base station performs non-coherent demodulation.
For most radiocommunication systems, multiple propagation paths between the transmitter and receiver stations constitute a difficult problem, especially because they cause signal fadings (Rayleigh fading). An important advantage of CDMA is to reduce the disadvantages relating to multiple paths, owing to the spreading of the signal spectrum.
With CDMA, it is even possible to take advantage of the existence of multiple paths to improve the reception performances, by means of spatial diversity techniques. To this end, a rake receiver is used, having several arms each performing reception of the same signals according to a selected propagation path. Each path is identified by a delay which is applied to the spreading sequence for determining a correlation between the received signal and the thereby delayed sequence. The correlations thereby obtained in different arms of the receiver can then be combined to restore the transmitted information bits.
This spatial diversity technique is explained in U.S. Pat. No. 5,109,390. In the system described in the latter document, one of the arm of the rake receiver is used for scanning the time domain to search for new propagation paths. The purpose of such search is to select, for the reception, the paths/delays for which the energy of the received signal after correlation is the most important. The delays are sequentially tested in a preset time window, by determining their associated energy. Each tested delay corresponds to an offset by a integer number of chips of the spreading sequence, which is applied thereto for calculating the correlation. The time window of the tested delays typically has a duration of about 100 .mu.s, which represents a propagation over about 30 km. Once a delay has been selected and allocated to one arm of the rake receiver, the corresponding path is tracked by means of a tracking loop, of the Costas loop-type, included in this arm. An example of such loop is described in the article "Theory of Spread Spectrum Communications - A tutorial" by R. L. Pickholtz et al., IEEE Trans. on Communications, Vol. COM-30, No. 5, May 1982.
The method of sequentially exploring the paths is reliable, but the applicant has observed that it does not take into account in an optimal way the characteristics of the propagation channel. Indeed, it leads to often test delays corresponding to improbable propagation paths. A consequence is that the selection of a "good" path, i.e. a path for which the received signal has a satisfying energy, is, on average, a relatively long process.
Another observation of the applicant is that, if it were possible to substantially reduce the mean time required for selecting a "good" path, it would be possible, in certain cases, to dispense with the tracking loops usually required in each arm of the rake receiver. This would lead to some loss on the level of the demodulated signal, because the synchronization of the sequences would no more be achieved with a definition finer than the duration of one chip. However, this loss would not be greater than 3 dB, because the worst case would correspond to a time offset by half a chip duration, whereby the instantaneous correlation would become zero when there is a transition in the signal, i.e. for one chip out of two on average, so that the integrated correlation would be reduced by a factor 2. This loss limited to 3 dB can be compensated for by the fact that a more rapid selection process allows for selection of the best paths in a more efficient way, and possibly by other means, for instance by adding one or more arms in the rake receiver. From the point of view of simplifying the demodulators, the suppression of the tracking loops would be an important advantage.
An object of the present invention is to propose a method allowing for a more rapid and efficient selection of the paths.